ci: avoid sending perf stat to Influx from forks

This patch is the follow-up for the commit
49946a72 ("ci: send perf statistics to InfluxDB").
Since secrets are unavailable for fork repositories, the sending step
fails due to a missed InfluxDB URL and token. This patch allows to run
this step only for on push events or PRs from the main repository
itself.

NO_DOC=CI
NO_CHANGELOG=CI
NO_TEST=CI